I got the oil idea from Chris Brady of Teres Audio. He tested different materials for his turntable. In my situation the oil is just barely enough to coat the sand when thoroughly mixed. Just enough to eliminate dust and scatter.
The "well" is an indentation for the sand that was cast into the concrete. Most people would use a box or tray to hold the sand. With concrete and granite the damping from sand is essential as concrete and granite really are not good by themselves. Massive and stiff but poorly damped, they ring real bad. That's why nothing sits directly on concrete or granite, and not even directly on the sand.
That rack was built around 2003. The sand at the top has a 150 lb granite plate sitting on it. The sand at the bottom has four MDF footers supporting a phono stage. Neither has shifted at all in what is now nearly 20 years.
The "well" is an indentation for the sand that was cast into the concrete. Most people would use a box or tray to hold the sand. With concrete and granite the damping from sand is essential as concrete and granite really are not good by themselves. Massive and stiff but poorly damped, they ring real bad. That's why nothing sits directly on concrete or granite, and not even directly on the sand.
That rack was built around 2003. The sand at the top has a 150 lb granite plate sitting on it. The sand at the bottom has four MDF footers supporting a phono stage. Neither has shifted at all in what is now nearly 20 years.